A typical power converter--a rectifier, generally followed by a power conditioner--which operates off of A.C. lines directly rectifies the voltage and stores energy on large input capacitors. As a consequence, it draws current from the A.C. lines in narrow but large current pulses, thereby yielding a poor power factor.
The power factor (P.F.) is the preferred method of measuring the efficiency of power passing through a point in a power distribution system. The power factor is the ratio of the average power, or true power, measured in watts, to the apparent power, measured in volt-amperes, drawn by a circuit. It is expressed as follows: ##EQU1## Typically, the power factor measure is applied to A.C. distribution systems in which the voltages and currents are substantially sinusoidal, though usually not in phase. In such systems, the power factor is simply calculated as the cosine of the phase angle between the current and the voltage.
Power is distributed most efficiently when the actual power delivered to a load equals the product of the input RMS voltage and current, i.e., when the power factor equals 1. However typical power factor values for power converters range from about 0.75 to less than 0.5.
Low power factor is compensated for by high current drawn by the converter in order to supply sufficient power to a load. Undesirable consequences of low power factor include (a) increased impendance losses, (b) the need for larger-capacity and more robust A.C. power distribution system components (e.g., circuit breakers, transformers, and wiring) that are capable of handling the power converter's high RMS current demands, (c) the need for larger and more robust rectifier diodes, storage capacitors, and wiring in the power converter to handle the power surges, and (d) greater difficulty and expense of meeting safety (e.g. Underwriter's Laboratories) requirements. Not only is the cost of power distribution increased thereby, but the cost of the power itself may be increased, because some users must pay not on the basis of the actual power consumed but on the basis of apparent power consumed. Clearly, then, it is desirable to improve the power factor of power converters.
The prior art does provide circuits for power factor compensation. For example, W. Shepherd and P. Zand suggest certain such circuits in Ch. 11 of their book Energy Flow and Power Factor in Nonsinusoidal Circuits, Cambridge University Press, 1979. Most are shunt circuits, both linear and nonlinear. These shunt circuits tend to be only marginally effective in improving the power factor. Several series-compensation schemes are also described, but they are complex, costly, and often unreliable active networks that supply the harmonic frequency content of the rectified load current so that the A.C. lines have to supply only the current at the fundamental frequency.
A compensation circuit that avoids the disadvantages of those proposed by Shephard and Zand is presented by G. J. Scoles in Ch. 18 of the Handbook of Rectifier Circuits, John. Wiley & Sons, 1980. He describes a "tuned bridge rectifier" in which an L-C circuit is placed in series between the A.C. lines and the rectifier, which L-C circuit is specifically tuned to resonate at the A.C. line frequency. This circuit has serious drawbacks of its own, however. With variations in load on the rectifier, the rectified voltage varies significantly--on the order of 30% between no load or light load and normal load (of about .pi./2.sqroot.LC)--which is an unacceptably high variance for most applications. At high loads, and particularly during start-ups, overloads, or short-circuits, the voltages and the current peaks across both the inductor and the capacitor of the L-C circuit get very large and are likely to damage both the inductor and the capacitor unless very robust, and hence very expensive, components are used that can withstand the surges and avoid breakdown.
A better solution than those hitherto proposed by the art is therefore required for improvement of power factor of power converters.